This invention relates to a method for an exchange of catalysts. Detailedly, the invention relates to a method for an exchange of catalysts in a carbon steel shell and tube type reactor, which method is fit to manufacture an objective product of high quality and low in its impurity content. More detailedly, the invention relates to a method for an exchange of catalysts in a carbon steel shell and tube type reactor, which method is fit to prepare an objective product of high quality and low in impurities such as aldehydes through catalytic gaseous oxidation of a hydrocarbon compound.
Manufacturing technology of ethylene oxide has been already advanced in pretty high level, but increasing its selectivity even if the degree is as small as on the order of 1 to 2% is industrially desirable in view of the large production scale of ethylene oxide. It is also industrially desired to manufacture of ethylene oxide of high quality by lowering the content of impurities such as aldehydes which are difficult to separate and remove by distillation.
For manufacture of ethylene oxide by catalytic gaseous oxidation of ethylene a shell and tube type reactor is generally used. This shell and tube type reactor has hitherto been made of stainless steel, but, in recent years, inexpensive carbon steel reactors have come to be used.
However, when ethylene oxide is manufactured through gaseous oxidation of ethylene using a carbon steel reactor, the content of impurities such as aldehydes which is difficult to separate and remove by distillation or the like, sometimes, increases and ethylene oxide of high quality cannot be obtained.
The present inventors have made sequential researches into this problem, and as a result, they found that when rust is formed on the internal surface of the tubes (reaction tubes), impurities such as aldehydes are formed during the oxidation reaction, and the formation of this rust is due to water as dew formed on the internal surface of the tubes during the work of exchange of catalysts in the reactor, namely during the work of charging the catalyst or discharge the catalyst.
As methods for preventing rust formation due to dew formation during the catalyst exchange work, there can be considered a method which comprises shutting off contact between the internal surface of the tubes and the outside air by continuously sending dry air into the tubes during the work and preventing the outside air from coming in, but dew formation cannot sufficiently be prevented by this method. As another method, there is a method which comprises preventing formation of rust by periodically blowing dry air into the tubes to remove water as dew formed, but this method needs many times of blowing operation and dew formation cannot sufficiently be prevented thereby.
Problems to be Solved by the Invention
An object of the invention lies in improving disadvantages in the usual catalytic gaseous oxidation method using a carbon steel reactor by providing a method for exchange of catalysts fit for manufacturing an objective product of high quality and low in its impurity content. Another object of the invention lies in providing a method for effectively preventing dew formation onto the internal surface of the tubes in exchanging the catalyst in the carbon steel shell and tube type reactor.
Means for Solving the Problems
The present inventors found that, in exchanging the catalyst in the tubes, when, for example, the shell side is filled with water and the temperature of the water is maintained higher than the atmospheric temperature, the temperature of the internal surface of the tubes is maintained higher than the dew point of the atmosphere, and dew formation from moisture in the atmosphere and formation of rust can effectively be prevented.
Thus, according to the invention, a method for exchange of catalysts is provided which comprises, in conducting catalyst exchange work in a carbon steel shell and tube type reactor, maintaining the temperature of the internal surface of the tubes higher than the dew point of the outside air.
Embodiments of Practice of the Invention
Usually, catalyst exchange work is conducted in the atmosphere. The method of the invention is characterized in maintaining the temperature of the internal surface of the tubes higher than the dew point of the atmosphere at the time of work. More specifically, the method of the invention is characterized in maintaining the temperature of the internal surface of the tubes higher than the temperature of the outside air.
In the invention, for maintaining the temperature of the internal surface of the tubes higher than the temperature of the outside air, a heat transfer medium is charged at the shell side, and the temperature of this heat transfer medium is maintained at temperatures at least 2xc2x0 C. higher than the temperature of the outside air, more specifically at temperatures 2 to 40xc2x0 C. higher than the temperature of the outside air, preferably at temperatures 5 to 20xc2x0 C. higher than the temperature of the outside air.
As the above-mentioned heat transfer medium, there can be used not only air or water but also kerosine, tetralin, diphenyl ether, Dowtherm(copyright) (trade name), etc.
In an embodiment of the invention, air of a temperature at least 2xc2x0 C. higher than the temperature of the outside air is introduced into the shell side. In another embodiment of the invention, the shell side is filled with water, and the temperature of the water is maintained at least 2xc2x0 C. higher than the temperature of the outside air. In view of easiness of handling, the latter embodiment using water is preferred.
The work of catalyst exchange itself can be carried out, with no particular limitation, according to a conventional method. For example, the catalyst is taken out from the inside of the tubes, and, if necessary after dry air is blown, a catalyst is charged inside the tubes. It is preferred to carry out the method of the invention at the time of the work of charge and/or discharge of the catalyst, particularly at the time of the work of charge and discharge.
As reaction using a carbon steel shell and tube type reactor, besides manufacture of ethylene oxide by oxidation of ethylene, there can be mentioned oxidation reaction and oxidative dehydro-genation reaction for manufacture of acrylic acid, maleic acid, phthalic anhydride, maleic anhydride, acrylonitrile, etc., and the like. The method of the invention can be applied to catalyst exchange in carbon steel shell and tube type reactors used in these reactions.
Dew formation onto the internal surface of the tubes at the catalyst exchange work is particularly frequent in the summer season and the rainy season when it is in high temperature and high humidity. Therefore, the method of the invention is particularly suitably used at catalyst exchange under high temperature and high humidity.
Effect of the Invention
According to the invention, dew formation at work of exchange of catalysts and subsequent formation of rust can efficiently be prevented by a convenient method. Thereby, the problems of formation of impurities due to the formation of rust, etc. are solved, and, for example in manufacture of ethylene oxide through oxidation of ethylene, ethylene oxide of high quality can be obtained.
In addition, according to the invention, it can effectively be prevented that inconveniences such as lowering of catalyst performance, particularly lowering of selectivity in oxidation reaction occurs by contact of water as the formed dew onto the catalyst.
Thus, by carrying out catalyst exchange according to the method of the invention, an objective oxidation product of high quality can be manufactured stably over a long period.